This invention relates generally to waveguide terminations and more particularly to high-power waveguide terminations.
As is known in the art, a waveguide termination or load is a one-port device that is commonly used to absorb electrical power incident upon it. Ideally, it is desired to have the impedance of the termination be equal to the characteristic impedance of the transmission media to which it is coupled to so that all of the incident power is absorbed.
Waveguide terminations generally include a lossy, usually resistive element disposed within the inner portion of the waveguide which absorbs the electrical energy and a shorting plate disposed at one end of the waveguide for preventing radiation leakage out of or into the waveguide structure. The other end of the waveguide is open and generally includes a flange. The flange includes mounting holes for allowing the connection of the termination to other waveguide components. To minimize reflections at the joint, the mating surfaces must be clean and flat and the adjoining waveguides must be properly aligned. The flanges are bolted together so that the mating surfaces make good ohmic contact, particularly at points along the broad walls of the waveguide, where longitudinal currents flow. In operation, electromagnetic energy enters the flanged end of the waveguide and is absorbed by the lossy element.
In high power applications, an effective connection between waveguides may be provided by using a choke flange. The cover-to-choke flange configuration is preferred in high power applications because the ohmic contact occurs at a minimum current point, thus arcing is avoided even if the contact is imperfect and erratic. The principle of operation of a choke flange is similar to that of a noncontacting short. The choke flange typically includes a circular groove disposed at a distance from the center of the waveguide and a transformer line section coupled to the groove which provides in combination a low impedance at the waveguide wall, thus providing continuity of longitudinal current flow between the waveguides.
There are a wide variety of waveguide terminations having elements with different sizes, shapes and material compositions generally dependent on the particular application and the frequency of operation.
One waveguide termination used when low standing wave ratios are required is the tapered load. The tapered load generally has a lossy element disposed within the waveguide which is gradually tapered to minimize wave reflections. The length of the taper for the lossy element is generally required to be at least a few wavelengths long at the lowest frequency of operation. The taper of the lossy element generally begins in intimate contact with the narrow walls of the guide where the electric field is negligible. This configuration minimizes the possibility of electrical arcing between the narrow walls of the waveguide at high power levels.
In higher power applications, forced air or liquid cooling may be required. In applications using air cooling, the waveguide housing generally includes finned portions for providing a greater surface area for radiating heat conducted from the tapered load to the waveguide walls and an optional fan for carrying away heated air from the finned portions.
Liquid cooled tapered load waveguide terminations generally include channels carrying a circulating coolant which is rigidly fixed to the waveguide housing for transferring heat out of the termination assembly. Because the load element is required to withstand high temperatures, ceramic based absorbing materials are commonly used. Although ceramic materials are generally refractory, their dielectric constant and loss tangent characteristics are also relatively temperature sensitive. Therefore, for very high power applications cooling methods are generally required.
A waveguide termination used in very high power applications is the ceramic block window load. One type of ceramic block window load includes a hollow waveguide having a fluid-tight coolant chamber portion disposed within an end portion of the waveguide and adjacent to the shorting plate of the termination. The liquid coolant acts as the resistive element and provides the lossy medium required for absorbing the electromagnetic energy. The coolant chamber portion generally includes a partition or baffle disposed generally along the central axis of the chamber for providing a channel for allowing the coolant to enter one side of the chamber and to exit through the other side. The ceramic window load further includes a slab fabricated from ceramic or other suitable refractory material selected to have a dielectric constant intermediate to the dielectric constants of air and the liquid coolant. The ceramic slab is generally brazed within the waveguide between the air filled waveguide and coolant filled chamber using conventional brazing techniques and provides a wall which is somewhat transparent to the electromagnetic energy. Although some of the electromagnetic energy is absorbed and dissipated within the ceramic slab window, it is generally desired that as much of the energy incident upon the load be dissipated in the coolant medium. The ceramic window load of this type may, in addition, further include one or more matching elements for providing an impedance match between the characteristic impedance of the air-filled waveguide and the energy absorbing coolant chamber. The impedance and the position within the waveguide will generally determine whether the elements are capacitive or inductive elements. Because the matching elements are generally frequency sensitive and although impedance matching elements increase the amount of energy delivered to the coolant at one frequency, the ability to absorb energy at other frequencies is typically degraded. In effect, the matching element generally provides a waveguide termination with a narrow bandwidth frequency response.
Another problem with the ceramic window load is that, as energy is absorbed by the coolant, the temperature of the coolant increases. This rise in temperature changes the dielectric constant and loss tangent characteristics of the coolant such that the effectiveness of any impedance matching elements is substantially reduced.
In addition, manufacturing costs for ceramic window water loads are relatively high. This is generally attributed to the brazing operations required for providing the ceramic slab in the waveguide and for providing the fluid-tight coolant compartment to the waveguide.
Further, in applications where the waveguide has corners, such as in a rectangular waveguide, air bubbles can accumulate in the corners of the coolant chamber such that the impedance of the energy absorbent chamber fluctuates. This fluctuation may reduce the amount of energy provided to the resistive element of the termination.
Another waveguide termination commonly used in high power applications is the glass water load. The glass water load includes a hollow glass tube having a uniform cross-section inserted through the narrow walls of the waveguide at a shallow angle. Circulating water or other suitable coolant is then passed through the tube for dissipating the incident power. Configured in this way, the reflections are minimized and accordingly the standing wave ratio is relatively low. The amount of power that can be dissipated in such a water load is related to the type of coolant used, the flow rate of the coolant, and the cross-sectional area of the tube.
One problem with the glass water load waveguide termination is that because the glass tube is inserted at a shallow angle, the tube is generally required to be quite long which presents problems of mechanical fit in applications where there is limited space. Also, in applications where a very low VSWR is required, the cross-sectional area must be kept to a minimum, which subsequently limits volume flow and power handling capability. Further, in applications where the waveguide termination may be subjected to mechanical shock the fragile glass tube can be easily broken.
In all of the above described high power waveguide terminations, voltage standing wave ratios of at least 1.05:1 are achievable, but usually at the expense of providing frequency sensitive tuning structures or exhaustive empirical attempts at repositioning the energy absorbent element. However, even in situations where an optimum position of the absorbent element is determined and can be repeated, an increased VSWR can result from deviations in the internal dimensions of the waveguide and the absorbent element during the manufacturing process of these elements, nullifying the determined optimum position. Refining the manufacturing specifications to require very high tolerance piece parts will correspondingly increase manufacturing costs. Moreover, an increased VSWR can also be caused by the lateral displacement between the two flanged assemblies.